Electrolytic capacitors, in which a gap is filled with an electrolyte in order to closely contact a dielectric film of a positive electrode with an opposing electrode, include a non-solid electrolytic capacitor in which an electrolyte is a liquid, a solid electrolytic capacitor in which an electrolyte is a solid, a hybrid type electrolytic capacitor in which an electrolyte is in a liquid and in a solid, and a bipolar electrolytic capacitor in which a dielectric film is formed on both electrodes. An electrolytic capacitor is formed by impregnating a capacitor element in an electrolyte, the capacitor element has a configuration in which a positive electrode foil in which a dielectric film is formed on a valve metal foil, such as aluminum, and a negative electrode foil which is made of a metal foil of the same metal or a different metal are positioned to face each other, and a separator is interposed between the positive electrode foil and the negative electrode foil.
The electrostatic capacitance of an electrolytic capacitor is proportional to a surface area of the dielectric film. In general, a surface enlargement process, such as etching, is performed on the electrode foil of an electrolytic capacitor, and a chemical treatment is performed on the surface enlarged part, which was subjected to the enlargement process, to have a dielectric film with a large surface area. In recent years, in order to further increase the electrostatic capacitance of an electrolytic capacitor, enlargement has progressed from the surface of the electrode foil to a deeper part.
In other words, in an electrolytic capacitor, the core part of the electrode foil is tending to be thinner. The surface expansion part having the dielectric film has flexibility and stretchability lower than the core part. Therefore, an electrode foil that had the surface enlargement of the dielectric film has decreased flexibility and stretchability due to thinning of the residual core part having high flexibility and stretchability.
Here, as an electrolytic capacitor using the electrode foil, a form of a winding capacitor may be used in order to reduce the size and increase a capacitance. The capacitor element of a winding capacitor is obtained by laminating the positive electrode foil and the negative electrode foil with a separator therebetween and winding them in a cylindrical form. In recent years, measures for increasing a surface area of a dielectric film have caused big problems with respect to winding properties of a winding capacitor.
That is, as shown in FIG. 10, when surface enlarged part 103, which surface enlargement was performed to, is chemically treated to form a dielectric film 105, the flexibility and stretchability of an electrode foil 101 decrease. Accordingly, the electrode foil 101 with decreased flexibility and stretchability may not be able to be deformed into a bow, and it may be difficult to curve smoothly and wind the electrode foil 101, bending some parts while winding. In particular, in the worst case, concentration of the bending stress during the winding may cause a crack 104 that breaks a core part 102. The crack 104 that breaks the core part 102 may induce bending of the electrode foil 101.
When the electrode foil 101 is wound with some parts bent, the diameter of the capacitor element increases. Therefore, in order to maintain the electrostatic capacitance of the winding capacitor, the size of the winding capacitor increases. Alternatively, in order to maintain the diameter of the winding capacitor, the electrostatic capacitance of the winding capacitor decreases. Otherwise, this is regarded as a defective product and the yield will deteriorate.